Fibers and Polymers 2014, Vol.15, No.3, 431-436
DOI 10.1007/s12221-014-0431-5
Preparation of Carboxymethyl Cellulose Superabsorbents from Waste Textiles Hosein Bidgoli, Akram Zamani1, Azam Jeihanipour2*, and Mohammad J. Taherzadeh School of Engineering, University of Borås, Borås, Sweden Chemical Engineering Department, Isfahan University of Technology, Isfahan 84156-83111, Iran 2 Faculty of Advanced Sciences and Technologies, University of Isfahan, Isfahan 81746-734, Iran (Received December 17, 2011; Revised October 23, 2012; Accepted March 3, 2013)
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Abstract: Production of superabsorbent polymers from cotton and viscose waste textiles was investigated. The cellulose wastes were carboxymethylated, crosslinked by divinylsulfone, and then converted to superabsorbent material using airdrying, freeze-drying, or air-drying after phase inversion. The separation of cellulose from synthetic polymers in the textile (polyester) was carried out by direct dissolution of cellulose in N-methylmorpholine-N-oxide (NMMO), or separation by dissolution in water after carboxymethylation of the textiles. The progress of the carboxymethylation reaction was evaluated by measurement of the degree of substitution (DS) of carboxymethyl cellulose (CMC). The DS values of 0.50-0.86 confirmed the prosperous substitution of hydrophilic carboxymethyl groups into the cellulosic chains. The water binding capacity and the swelling rate of the superabsorbents prepared under different conditions were measured. Under the best condition the superabsorbent obtained from waste textiles showed an ultimate water binding capacity of 541 g/g which was notably higher than that of the reference superabsorbent derived from cotton linter (470 g/g). The amount of absorbed water by this product exceeded that of the reference sample after 60 min immersion. Keywords: Carboxymethyl cellulose, Cellulose separation, Superabsorbent polymer, Waste textiles
binding capacity, biocompatibility, and biodegradability [1517]. These superabsorbent polymers are usually produced by slight crosslinking of a mixture of CMC and hydroxyethyl cellulose (HEC). In this process, divinylsulfone, a di-functional molecule, is used as a crosslinker [18,19]. The superabsorbent polymer obtained can be used as an alternative for commercial synthetic products such as polyacrylates [19]. Aiming at finding a new way of recovery of waste textiles to a value-added product, current work focused on studying the possibility of conversion of cellulosic part of the waste textiles to superabsorbent polymers. Different methods of separation and purification of cellulose along with conversion of this biopolymer to superabsorbent polymers were investigated.
Introduction The current methods of disposal of waste textiles are generally landfilling and incineration [1,2]. Environmental concerns about growing landfills in addition to the high potential of waste textiles for different applications has opened up a new area on waste textiles refinery in recent years [3-5]. Production of ethanol [6], and superabsorbent polymers [7] from cellulose-based waste textiles along with production of activated carbon from acrylic acid textile wastes [8] are three examples of successful utilization of this type of waste for production of value-added products. Attempts for refining waste textiles as a rich source of energy and materials, will consequently lead to lower growth of landfills and acquire remarkable economic developments. Cellulose-based materials such as cotton and viscose comprise a major fraction of fiber markets [9]. Cellulose is a linear naturally occurring polysaccharide with excellent properties and numerous applications [10]. After purification, it can be industrially converted to numerous derivatives such as cellulose esters and cellulose ethers [11]. Among different cellulose derivatives, carboxymethyl cellulose (CMC) is the most industrially applicable cellulose ether and has the highest commercial demand [12]. This anionic water soluble polysaccharide has wide applications in food, pharmaceutical, cosmetic, and medical industries [13]. In addition, CMC is a promising material for production of bio-based hydrogels, due to its high water solubility [14]. In recent years, cellulose-based superabsorbent polymers have attained increasing attention due to their high water
Experimental Materials Cotton linter and two types of cloths (used as waste textiles) of (A): an orange blend of 50 % polyester and 50 % cotton (cotton textile), and (B): a blue blend of 40 % polyester and 60 % viscose (viscose textile) were purchased from a local shop in Borås, Sweden. Hydroxyethyl cellulose (viscosity of 145 mPa·s of 1 % solution in H2O at 20 oC) was supplied by Sigma-Aldrich (Germany). Commercial 50 % aqueous solution of N-methylmorpholine N-oxide (NMMO) was provided by BASF (Ludwigshafen, Germany). Separation of Cellulose from Waste Textiles The cotton and viscose were separated from their blends in waste textiles, applying two different methods. The first method was based on dissolution of cellulose in NMMO, a
*Corresponding author:
[email protected] 431
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cellulose solvent, and its simultaneous separation from the polyester. The second method was aimed at direct carboxymethylation of cellulose in the textiles followed by dissolution of the sodium carboxymethyl cellulose (CMCNa) in water. Separation Using NMMO A former method developed for separation of cellulosic fractions of waste textile with minor modifications was used [3]. Briefly, NMMO (50 % solution in water) was concentrated up to 85 % in a rotary vacuum evaporator (Laborota 20, Heidolph, Schwabach, Germany) equipped with a vacuum pump (PC 3004 VARIO, Vacuubrand, Wertheim, Germany). Then, 50 g of textiles (in square pieces of 3×3 cm2) were mixed with 950 g of the concentrated solvent in a 2 l beaker equipped with an impeller and an electrical motor, and placed in an oil bath at 120 oC for 3 h. The dissolved cellulose was then separated from the insoluble polyester by passing the mixture through a kitchen metal screen (1 mm pore size). In order to regenerate the dissolved cellulose, the cellulose solution was gently added to a beaker containing 3 l of boiling water, while the suspension was mixed continuously. The regenerated cellulose was separated from the water and NMMO by vacuum filtration. This cellulose was finally washed 4 times with hot water and kept at 4 oC until use. The remaining water was then removed from the wet regenerated cellulose using different methods of (a) freezedrying, (b) air-drying at ambient temperature, and (c) airdrying after extraction of water from the gel by acetone. In the third method, 3 g of the wet cellulose was mixed with 30 ml 2-propanol and stirred vigorously until a smooth suspension was obtained. Then, the liquid phase was separated by vacuum filtration. This process was repeated 3 times and the precipitated cellulose was spread on a petri dish at ambient pressure and temperature for 24 h for complete drying. The dried cellulose was milled in a ball mill (30 Hz frequency) for 30 s before carboxymethylation. Separation by Direct Carboxymethylation of Waste Textiles In this method, the textile pieces (3×3 cm2) were cooled
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down by soaking in liquid nitrogen (−196 oC) to get brittle textiles [20]. Then, the pieces were milled in a ball mill (Retsch GmbH, MM400, Germany) with a frequency of 30 Hz for 4 min. The obtained textile powder was then directly used as a source of cellulose in carboxymethylation reaction according to the next section. After carboxymethylation, 2 g of the product was grinded in the ball mill and mixed with 60 ml of water until a homogenous suspension was obtained. The mixture containing dissolved CMC was afterwards centrifuged for 5 min at 10,000×g. The precipitate was washed twice with 4 ml water to dissolve the remaining CMC and centrifuged. Finally, the supernatant was dried at 60 oC to get CMC. Carboxymethylation of Cellulose The carboxymethylation reaction was carried out according to a previous method with slight modifications [21]. In brief, 2 g cellulosic material (cotton linter, NMMO separated cellulose, or textile powder) was weighed and suspended in 53.3 ml of 2-propanol. Then, 6.67 ml of 40 % aqueous solution of NaOH was added to the mixture over 30 min under vigorous mixing. The stirring continued for 1 h. Afterwards, 4.8 ml of 50 % solution of monochloroacetic acid in 2-propanol was added to the mixture over 30 min. Then, the container (100 ml beaker) was covered with an aluminum foil and kept at 55 oC for 3.5 h with continuous stirring. At the end, the liquid was drained off and the solid was mixed with 20 ml of 70 % methanol and neutralized to pH 7 by addition of 90 % acetic acid. Finally, the product was separated by vacuum filtration and resuspended in 40 ml of 70 % ethanol for 10 min for desalting and dewatering, and then the CMC was recovered by filtration. The filtration and mixing with ethanol was repeated 6 times. Finally, the product was dried at 60 oC and grinded to a smooth powder by ball-milling for 10 seconds at 30 Hz. The dried NMMO separated cotton and viscose celluloses were also used to produce CMCs. Moreover, two CMCs were prepared by direct carboxymethylation of the blend of polyester and cotton. A reference CMC was produced by carboxymethylation of cotton linter. All the CMCs along
Table 1. Different CMCs produced and their source of cellulose, methods of separation, and methods of drying of cellulose Superabsorbent no. Source of cellulose Method of separation Method of drying a 1 Cotton linter 2 Cotton textileb NMMO Air-dried 3 Cotton textile NMMO Freeze-dried 4 Cotton textile NMMO Air-dried after phase inversion with 2-propanol 5 Cotton textile Direct carboxymethylation 6 Viscose textilec NMMO Air-dried 7 Viscose textile NMMO Freeze-dried 8 Viscose textile NMMO Air-dried after phase inversion with 2-propanol 9 Viscose textile Direct carboxymethylation a b c Reference, A blend of 50 % polyester and 50 % cotton, and A blend of 40 % polyester and 60 % viscose.
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Figure 1. Conductimetric titration curve of a solution comprising CMCNa acquired from cotton textile by direct carboxymethylation (No. 5) titrated with 0.33 N HCl.
with their cellulose sources, methods of separation and methods of drying are summarized in Table 1. Determination of Degree of Substitution (DS) The degree of substitution of the CMCs produced was measured by conductimetric titration according to Browning [21] with minor modifications. Briefly, 0.3 g of dried CMC, were added to 15 ml 70 % methanol and stirred for 10 min. Then, 200 ml deionized water and 3 ml 0.5 N NaOH were added to the mixture with continuous stirring until complete dissolution or dispersion of CMC. Afterward, the solution was titrated with 0.33 N HCl. A typical plot of conductivity versus volume of the titrant is shown in Figure 1. The degree of substitution (DS) of each CMC was calculated according to equation (1) [21]: 0.162 × A DS = ---------------------------------1 – ( 0.080 × A )
(1)
where, A is the meq (milliequivalent) of total carboxyl groups per gram of the CMC that was calculated by the following equation: ( V 2 – V1 ) × N A = ---------------------------W
(2)
where V1, V2 are the volumes of titrant at two intersections (Figure 1), N is the normality of HCl, and W is the weight of the CMC in grams. Preparation of Superabsorbent Polymer from CMC Synthesis of superabsorbent polymer in this work was carried out according to a previous report with slight modifications [19]. CMC and HEC with 3:1 ratio were stirred gently in a mixture of water and divinylsulfone (0.04 mol/l) at room temperature. Stirring was continued until a clear solution with final polymer concentration of 2 % was obtained. The presence of HEC in the reaction was essential since it promoted the intermolecular rather than intramolecular
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crosslinking [19]. The concentration of divinylsulfone was chosen as 0.04 mole/l of solution. The crosslinking reaction was initiated by addition of 1 M KOH as the catalyst (up to a final KOH concentration of 0.02 M). After a few minutes, the viscosity of the mixture was remarkably increased and a partially swollen gel was obtained within 1 h. The obtained gel was immersed in excess amounts of distilled water and filtered to desalt. This process was performed several times until the highest water binding capacity was reached (equilibrium condition). At this stage, the impurities remaining in the gel from the reaction mixture, e.g. KOH, unreacted divinylsulfone, and uncrosslinked polymers were likely to be totally washed out from the hydrogel. The product was a fully swollen purified hydrogel with the highest possible swelling capacity. The swelling capacity measured in this step named as initial water-binding capacity. Drying of the Hydrogels Drying of CMC hydrogels was carried out using air-drying at ambient temperature overnight, or solvent extraction followed by air-drying [19]. In the latter method, the gel particles were immersed in acetone for 5 h. Immediately after immersion, the water was released from the gel structure and consequently the gel was shrunk gradually and easily separated from the water using a metal sieve. The water content of the gel was totally replaced with fresh acetone by performing this process for three time. Then, the product was kept under vacuum at ambient temperature overnight to remove the remaining acetone. Measurement of Water Binding Capacity The superabsorbents were grinded to particles of less than 0.5 mm diameter. Then, 0.02 g of each sample was placed in a 50 ml beaker and immersed in excessive amount of distilled water for 1-48 h. The swollen gels were separated from the water using a metal screen and carefully weighed. The water-binding capacity of each product was calculated as (W2−W1)/W1, where W1 and W2 are weights of the dry and swollen superabsorbent, respectively. All experiments were performed at least in duplicate and the standard deviations of the results were less than 8 %.
Results and Discussion Carboxymethyl cellulose as an anionic raw material for production of hydrogels with high quality is used in both scientific and commercial areas [22]. Nowadays, purified pulp derived from natural biomass is the most important source for CMC [10,23]. The goal of the current study was introduction of waste textiles as an alternative source for production of carboxymethyl cellulose. As an example of applications carboxymethyl cellulose was used for production of superabsorbent polymers.
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Separation of Cellulose from Waste Textiles Two separations methods were applied to separate polyester from cellulose in textile wastes. In the first method, cotton or viscose were dissolved in NMMO and separated from the insoluble polyester of textiles. The dissolved cellulose was precipitated by adding water to the NMMO solution and filtered. The separated cellulose contained 85 % water. Using this wet form of the cellulose in carboxymethylation reaction, would subsequently lead to dilution of the reaction mixture and hence decreasing the reaction efficiency. Therefore, the water was removed prior to the carboxymethylation. In the second method, the carboxymethylation reaction and separation of polyester were carried out in one step of conversion of the cellulosic part of the textile to water soluble CMC. The dissolved CMC was then separated from the polyester particles by centrifugation. Determination of Degree of Substitution (DS) The progress of carboxymethylation reactions were evaluated by measuring the degree of substitution (DS) of the CMCs using conductimetric titration method and the results are shown in Table 2. In this method, the DS of each CMC was measured based on its corresponding titration curve (Figure 1). The volume between intersections of three straight lines defined by titration curve is the amount of acid required to convert the CMCNa to the free acid. It would therefore, correspond to the CH2COOH groups substituted in -OH position of cellulose chains in the course of carboxymethylation reaction. A good reproducibility was observed in the results, so that the standard deviation of DS values was less than 5 % under identical conditions. The lowest (0.50) and the highest (0.86) DS measured in this work corresponded to the CMC originated from cellulose separated from cotton textile by NMMO and air-dried (No. 2), and CMC acquired from cotton textile by direct carboxymethylation (No. 5), respectively (Table 2). Generally, the DS values of the CMCs obtained from cellulose separated by NMMO were lower than that of the CMCs obtained from direct carboxymethylation of textile. This difference may be Table 2. Degree of substitution of different CMCs measured by conductimetric titration method CMC No. 1a 2 3 4 5 6 7 8 9 a
Reference.
Degree of substitution (DS) 0.75 0.50 0.66 0.65 0.86 0.59 0.53 0.69 0.70
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related to the side-effects of water removal from cellulose after separation by NMMO. The drying method of cellulose had a crucial impact on the efficiency of carboxymethylation reaction and consequently DS of CMC (Table 2). The DS value after performing the carboxymethylation reaction on the regenerated cotton was maximum when dried by freezedrying method (DS=0.66). However, the highest DS of the regenerated viscose (0.69) was obtained when extraction with 2-propanol was applied. The drying step maybe ended with collapsing the cellulose structure and as a result weakens the diffusion of NaOH and monochloroacetic acid into the polymer network and consequently decreases the reaction efficiency. On the other hand, when the direct carboxymethylation of textile was applied as the separation method, only a part of CMC which is water soluble (i.e. DS>0.4 [12]) was separated from the textiles and the insoluble part (DS
